1. Field of the Invention
The present invention relates to power ramping in a communications transmitter.
2. State of the Art
High quality RF (radio frequency) signals must ramp quickly from a condition of minimal output power to a condition of information-bearing modulation at a specified output power and back down to the condition of minimal output power. Such power ramping capability, illustrated in FIG. 1, is required for transmitters in many time division multiple access (TDMA) communication systems. Example systems include those specified by the GSM and ANSI-136 standards, and combinations of the same (so-called multi-mode systems).
A fundamental requirement of these transmitters is that the acts of ramping up and ramping down must not violate specified limits on peak power in spectral bands away from the assigned RF channel (e.g., bands that would be allocated to other transmitters); the associated measurement is called the transient spectrum in some systems or the transient adjacent channel power (transient ACP) in others.
Present power ramping techniques must be tailored for each modulation type, and typically require unit-by-unit calibration (at least in the case of typical GMSK transmitters and conventional multi-mode transmitters). Even so, transient ACP performance is usually very sub-optimal.
The present invention is applicable to both conventional (I/Q) and polar modulation architectures. Polar modulation architectures, and similar architectures in which separate amplitude and phase paths are provided, are described, for example, in U.S. Pat. Nos. 6,191,653, 6,194,963, 6,078,628, 5,705,959, 6,101,224, 5,847,602, 6,043,707, and 3,900,823, as well as French patent publication FR 2768574, all of which are incorporated herein by reference.